Function liquidity_step 

pub fn liquidity_step(pool: u64, inflow: u64, drain_percent: u64) -> u64

One discrete Euler step of the liquidity ODE dL/dt = inflow − drain·L: constant inflow replenishment minus drainage proportional to the current pool (drain_percent% per step). Saturating; zero-heap. The steady state is L* = inflow / drain.